Sensor for measuring a magnetic field

ABSTRACT

The invention relates to a sensor for measuring a magnetic field. The inventive sensor has a high level of measuring sensitivity compared to a Hall probe, comprising several electrically semiconductive layers. The layers are arranged in the form of a power diode connected in the reverse direction, consisting of an anodelayer, a cathode layer and an intrinsically conductive layer enclosed between the two. The anode layer is subdivided by insulation sections into several anode layer areas, these areas being insulated from each other. The cathode layer has an injector area on the areas opposite the insulation sections which is oppositely doped. An electron beam is formed between the injector area and the anode by applying an injection voltage to the injector area. The electron beam is distributed across the areas of the earthed anode layer areas in the form of uniform partial currents. The electron beam is diverted by a magnetic field which forms in the intrinsically conductive layer, resulting in partial currents of different strengths on the earthed anode layer areas. The magnetic field can then be evaluated by measuring these differences in strength between the partial currents.

FIELD OF THE INVENTION

The present invention relates to a sensor for measuring a magnetic fieldwhich comprises several electrically semiconductive layers.

BACKGROUND OF THE INVENTION

Sensors for measuring a magnetic field are used in many fields today.They are, for example, used in automation, control and measuringtechnology in order to measure distances in a contact-free manner, toascertain speeds or to test positionings. In information technology,miniaturized magnetic field sensors read digital data which are, forexample, magnetically stored on hard drives. Sensitive magnetic fieldsensors are used in non-destructive material testing in order to detectmaterial defects. In medical diagnostics, highly sensitive sensors areused to quantitatively record biomagnetism.

The sensors must meet quite a few different requirements correspondingto the diversity of applications. This can be accomplished with sensorswhich consist of different materials (such as metals, semiconductors orsuperconductors and which are based on different physical principles.

The individual sensors can thereby be roughly divided into the followingtypes:

field plates, Hall probes, magnetic diodes and magnetic transistorsconsisting of semiconductors;

magnetic resistive homogeneous or granular layers and layer systemsconsisting of metals and insulators;

SQUIDS consisting of superconductors

The types of sensors used most often are:

[1] field plates consisting of InSb with NiSb needles,

[2] Hall probes made of silicon or III/V semiconductors,

[3] magnetic resistive AMR sensors which consist of a homogeneous thinmagnetic metal layer of, for example, NiFe,

[4] GMR sensors which consist of at least two magnetic metal layers anda non-magnetic metal layer and

[5] SQUIDS consisting of the superconductors Nb or YBCO whose operatingtemperature is at 4K or 80K, respectively.

The sensitivity of the sensors vis-à-vis a magnetic field or a change inthe magnetic field increases continuously from the top to the bottom inthe above arrangements.

The following is noted briefly about the mode of operation of theindividual types of sensors:

The reason for the change in resistance in the field plates [1] and forthe Hall voltage [2] is the Lorentz force which is produced atelectrical charges when they move in the magnetic field.

In AMR sensors [3], the change in resistance is based on a magneticscattering of the conduction electrons which changes with the directionof the magnetizing relative to the direction of the current.

In the GMR [4], the action is again based on an anisotropic scatteringat the contact surfaces which depends on the angle that bothmagnetizations form with one another.

In the SQUIDS [5], the magnetic behaviour is determined by the principlethat the magnetic flow in a superconductive ring must be an integralmultiple of the elementary flow quantum.

SUMMARY OF THE INVENTION

The object of the present invention is to create a sensor for measuringa magnetic field which is very sensitive and, as a result, can alreadybe used to measure the smallest magnetic fields or to measure thechanges in magnetic fields. Moreover, it should be able to operate atroom temperature and not exhibit any hysteresis manifestations.

This object is solved with the sensor of the invention according toclaim 1 which consists of several electrically semiconductive layerswhich are arranged in the form of a diode or pin diode connected inreverse direction. This layer arrangement comprises an anode layer, acathode layer and an intrinsically conductive layer enclosed between thetwo. The anode layer is subdivided by insulation sections, for examplein the form of insulating strips, into several anode layer areas thatare insulated from one another. The cathode layer has an injector areaon the areas opposite the insulation sections which is oppositely doped.The anode layer and the cathode layer are connected with an inversevoltage, so that the layer arrangement is biased in reverse direction.An injection voltage is applied to the injector area in the cathodelayer. As a result, an electron beam is formed between the injector areaand the anode thereby that injected electrons move from the injectorarea on the cathode to the anode lying opposite thereto. A distributionof the electron beam takes place thereby due to thermal diffusion. Thisdistributed electron beam now uniformly strikes the individual anodelayer areas insulated from one another. The anode layer areas areindividually earthed by respective current measuring devices. Thus,depending on the number of anode layer areas, a corresponding part ofthe entire electron beam will strike every single anode layer area. Thispart of the electron beam can be measured in the form of a current withthe respective current measuring device.

If a magnetic field is now applied in the intrinsically conductive layerdiagonally to the direction of dispersion of the electron beam whichextends between injector area and anode layer, the Lorentz force is thenexerted on the drifting electrons which results in a diversion of theelectron beam. This diversion of the electron beam leads thereto thatother partial amounts of the entire electron beam now strike theindividual anode layer areas insulated from one another, which resultsin a corresponding change of the partial current of the anode layerareas. The magnetic field can be calculated from this change of thepartial currents.

BRIEF DESCRIPTION OF THE DRAWINGS

The construction and mode of functioning of the sensor according to theinvention for measuring a magnetic field will be described in greaterdetail in the following by way of two embodiments with reference to thedrawings, showing:

FIG. 1 a three-dimensional view of the sensor according to theinvention;

FIG. 2 a cross section of the sensor according to the invention withapplied voltage supply;

FIG. 3 a view as in FIG. 2 wherein, in addition, the course of theelectron beam and the magnetic field are shown; and

FIG. 4 an alternative embodiment of the sensor according to theinvention for measuring a magnetic field.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a three-dimensional view of the sensor according to theinvention for measuring a magnetic field. It consists of a layer packetof electrically semiconductive layers. The uppermost layer is thecathode layer 2 which, in this example, consists of a p⁺ doped layer.The lowermost layer is the anode layer 4 which, in this example,consists of an n⁺ doped layer. An intrinsically conductive layer 3 issituated between these layers.

The anode layer 4 is subdivided into several anode layer areas insulatedfrom one another. In the example shown here, the anode layer 4 issubdivided into two anode layer areas 4 a and 4 b. An insulation section5 is placed between the anode layer areas 4 a and 4 b for mutualinsulation of these areas, said insulation section 5 being configured inthe form of an insulating strip in this embodiment.

The cathode layer 2 has a so-called injector area 1 in the area oppositethe insulation section 5 of the anode layer 4. In this case, it has n⁺doping.

FIGS. 2 now shows how this layer packet of the sensor according to theinvention is electrically connected. The layer packet is shown in crosssection in this case. The p⁺ doped cathode layer 2, into which the n⁺doped injector area 1 is inserted in the middle, can be seen on the leftside. The two anode layer areas 4 a and 4 b, which are insulated fromone another by the insulation section 5, are shown on the right side.The intrinsically conductive layer 3 is found between anode and cathode.The individual anode layer areas 4 a, 4 b are each earthed via currentmeasuring devices 8 a and 8 b, schematically indicated here. The currentflowing through the current measuring device of the upper anode layerarea 4 a is thereby indicated with J₁, and the current flowing throughthe current measuring device of the lower anode layer area 4 b isindicated with J_(r).

An inverse voltage source 7 is now connected with the cathode layer 2,as a result of which the negative voltage −U₂ is here applied. Inconventional materials for the intrinsically conductive layer, wherebyit has a residual conductivity of about 10⁻⁴ Ω⁻¹ cm⁻¹ and a thickness of500 μn, the inverse voltage is about 100 V. An injection voltage source6 is now connected with the injector area 1, as a result of which thenegative voltage −U₁ is applied to this area. It is selected in such away that it is slightly lower than the cathode potential.

The function of the sensor according to the invention will now beexplained with reference to FIG. 3. FIG. 3 shows the same view as FIG.2. In addition, the path of the electron beam and the magnetic fieldhave now been drawn in.

By applying the inverse voltage U₂, an electrical field having thestrength

E=U ₂ /L  (1)

is configured between cathode layer 2 and anode layer 4. L therebydesignates the thickness of the intrinsically conductive layer 3.

By applying the injector voltage U₁, an electron beam 9 now formsbetween the injector area 1 and the anode layer 4 consisting of theanode layer areas 4 a and 4 b, thereby that electrons injected from theinjector area 1 drift to the anode layer 4. The drift speed V_(D) of theelectrons is given by the mobility μ_(e) of the electrons and by theprevailing electrical field E_(x):

V _(D)=μ_(e) E _(x)  (2)

Thus, the drift time t_(D) over the distance between cathode and anodeis:

t _(D) =L/v _(D)  (3)

During this time, the drifting electron beam 9 spreads due to thermaldiffusion. This is shown in FIG. 3 by an expansion of the electron beam9 on its path from the left cathode layer 2 to the right anode layer 4.There is a bell-shaped charge distribution as indicated by thecontinuous curve 11. The charge distribution on the anode layer therebyhas a variance <y²>^(½), which is calculated as follows:

<Y ²>^(½)=(D t_(D))^(½)  (4)

Using the Einstein correlation

μ_(e)=(q/kT)D  (5)

and with the previous equations (1) to (3), one obtains

<y ²>^(½)=(KT/qU ₂)^(½) L  (6)

wherein in these equations, k designates the Boltzmann constant, T thetemperature and q the elemental charge of the electrons.

At a room temperature of 300 K, with the inverse voltage of U₂=100 V andwith a thickness L of the intrinsically conductive layer of L=300 μm, anexpansion of the electron beam having the variance <y²>^(½)=4.8 μm isobtained and at a thickness L of 500 μm, an expansion with the variance<y²>^(½)=8 μm.

These quantities of the beam expansion determine the width of theinjector area 1 and the width of the insulation section 5. In fact,these may at the most exhibit the expansion <y²>^(½). However, this iseasy to attain with the aforementioned sample numerical quantities.

Since the injector area 1 of the cathode layer 2 is situated exactlyopposite the insulation section 5 of the anode layer 4 and the anodelayer 4 is subdivided into two equally large anode layer halves 4 a and4 b, this symmetrical arrangement results in two equally large currentsJ₁, and J_(r) in the anode layer halves 4 a or 4 b, respectively. Thesecurrents are measured with the current measuring devices 8 a or 8 b.

If a magnetic field 10 having the quantity B is now applied at rightangles to the direction of expansion of the electron beam 9 in theintrinsically conductive layer 3, then the electron beam is diverted.The Lorentz force which acts as an electric transverse field Ey isexerted onto the electrons drifting in the electron beam:

Ey=(μ_(e) B)E _(x)  (7)

This results in a diversion y_(B) for the electrons entering the anodelayer 4:

y _(B)=(μ_(e) B)L  (8)

In the example shown, this results in an upward shifted curve of thecharge distribution at the anode layer 4, as is indicated by the brokencharge distribution curve 12 in FIG. 3. As a result, the current J₁ ofthe anode layer area 4 a is enlarged and the current J_(r) of the anodelayer area 4 b is reduced. Thus, the symmetry of the current isdisturbed.

When the diversion y_(B) of the electron beam 9 on the anode layer 4becomes comparable with the variance <y²>^(½) of the electron beamdistribution, then the asymmetry, given by the current quotientsQ_(J)(B):

Q _(J)(B)=(J _(r) −J ₁)/(J _(r) +J ₁)  (9)

becomes clear and Q_(J)(B) almost assumes the value 1.

The magnetic field B₁ belongs to this:

B ₁=(1/μ_(e))(kT/qU ₂)^(½)  (10)

This equation no longer includes the drift distance of the electrons,i.e. the thickness L of the intrinsically conductive layer 3. Thecurrent quotient Q_(J)(B) has the value 0 if there is no magnetic fieldand it changes monotonously to the value −1 or +1 if the magnetic fieldchanges to the value B₁ in accordance with equation (10).

The field sensitivity of the sensor is described by the incline:

δQ _(J)(B)/δB=1/B ₁=μ_(e)(qU ₂ /kT)^(½)  (11)

A field sensitivity value of 6.2 T⁻¹ is obtained with the numericalvalues already noted above and the mobility μ_(e)=1000 cm² V⁻¹ s⁻¹ ofelectrons in the silicon. The corresponding quantity is, for example, inHall probes 0.1 T⁻¹. Therefore, an increase in sensitivity by the factor(qU₂/KT)^(½)=62 is obtained with the probe according to the invention incomparison to a Hall probe.

The anode layer 4 was subdivided into two equally large anode layerareas 4 a and 4 b, which are separated by a strip-shaped insulationsection 5, in the sensor shown in the aforementioned drawings. Theinjector area 1 of the cathode layer 2 opposite the insulation section 5thus also has a form like a longitudinal strip. Therefore, a sensorconstructed in this way can only measure the field component of amagnetic field which extends in longitudinal direction of the injectorarea (in FIG. 3, this is a field component of the magnetic field whichextends into the plane of the drawing or out of it).

FIG. 4 now shows an alternative embodiment of the sensor according tothe invention for measuring a magnetic field. In this case, the anodelayer is subdivided into four equally large anode layer areas 4 a, b, c,d. The corresponding insulation section 5 consists of two strips Sa and5 b crossing at right angles. The injector area 1 in the cathode layer 2is now in form of a square surface which is situated directly oppositethe point of intersection of the insulation sections 5 a and 5 b.

With this, it is possible to determine both components of a magneticfield at right angles to the direction of expansion of the electron beam9. The currents J₁, J₂, J₃ and J₄ of the anode layer areas 4 a, 4 b, 4 cand/or 4 d are also measured. To determine one of the magnetic fieldcomponents, the difference J_(r)−J_(u) is formed and to determine theother component, the difference J_(o)−J_(u) is formed. In this case, ifthe corresponding references of the currents are taken from FIG. 4:

J _(r) =J ₁ +J ₄  (11)

J ₁ =J ₂ +J ₃  (12)

J _(o) =J ₁ +J ₂  (13)

J _(u) =J ₃ +J ₄  (14)

If there is no magnetic field, then all four sum currents of theequations (11) to (14) are equal in the symmetrical arrangement shownand thus the difference currents J_(r)−J₁ and J_(o)−J_(u) are equal tozero.

What is claim is:
 1. Sensor for measuring a magnetic field, consistingof several electrically semiconductive layers, characterized thereinthat it comprises a packet of directly successive planar semiconductivelayers which form a diode connected in reverse direction consisting of acathode layer, an anode layer and an intrinsically conductive layerenclosed between the two, wherein the anode layer is subdivided intoseveral anode layer areas insulated from one another by insulationsections; the cathode layer which, in the area which is opposite theinsulation sections on the anode layer, exhibits an injector area thatis oppositely doped, the injector area in the cathode layer is connectedwith an injection voltage source; the cathode layer is connected with aninverse voltage source and the anode layer areas are each earthed viacurrent measuring devices.
 2. Sensor according to claim 1, characterizedin that the cathode layer consists of a p⁺ doped layer the anode layerconsists of an n⁺ doped layer, and the injector area consists of an n⁺doped layer.
 3. Sensor according to claim 1, characterized in that thecathode layer consists of an n⁺ doped layer the anode layer consists ofa p⁺ doped layer, and the injector area consists of a p⁺ doped layer. 4.Sensor according to claim 1, characterized in that the anode layer issubdivided into two parallel anode layer areas by a strip-shapedinsulation section.
 5. Sensor according to claim 1, characterized inthat the anode layer is subdivided into four parallel anode layer areasby two strip-shaped insulation sections that cross one another at rightangles.
 6. Sensor according to claim 1, characterized in that saidsensor is made from a high-quality semiconductor material with lowdoping and high mobility of holes and electrons.
 7. Sensor according toclaim 1, characterized in that it is made of silicon.
 8. Sensoraccording to claim 1, characterized in that said sensor is made of asemiconductive material that has elements from the III^(rd) main groupand elements from the V^(th) main group of the periodical system ofelements.